1. Field of the Invention
The present invention relates generally to air induction systems for providing increased airflow to the intake of an engine. More particularly, the present invention concerns a multi-phase centrifugal supercharging air induction system having a pair of superchargers that phase between serial and parallel operation to supply constant target boost to the intake over the entire rev range of the engine.
2. Discussion of Prior Art
Compressors that increase air flow to an engine (thereby increasing the power generated thereby) are known in the art and generally include positive displacement blowers, turbochargers, and centrifugal superchargers. In addition to increased air flow, all of these compressors desirably supply pressurized air to the engine—i.e., air under increased pressure relative to the normal atmospheric pressure of ambient air at the system's inlet. The pressurized air in combination with the increased air flow is commonly referred to as “boost.” Positive displacement (“PD”) blowers typically utilize a pair of intermeshing, counter-rotating figure-eight shaped impellers, or a screw-type impeller, driven off of the crankshaft to “push” air into the intake manifold thereby compressing the air. PD blowers supply fairly constant flow to the engine over varying pressure conditions and engine rpm. Turbochargers typically utilize an impeller driven by a turbine that is powered by the exhaust output of the engine to compress air for the engine. Turbochargers supply fairly constant boost to the engine only through the powerband—i.e., the engine is operating at fifty percent of its rev range or higher, a.k.a. “redline.” Centrifugal superchargers typically utilize an impeller powered by the engine's crankshaft to compress air for the engine. Centrifugal superchargers supply boost to the engine generally defined by a somewhat linear boost response curve wherein the boost supplied varies with engine rpm.
All of these prior art compressors are problematic and suffer from undesirable limitations. For example, PD blowers are inefficient, requiring an undesirable amount of engine horsepower to drive the impellers thereby effectively reducing any supplied boost, and are part-intensive, both of which increase the risks of catastrophic failure, such as through heat degradation. Additionally, the inefficiency of PD blowers worsens at higher engine rpms, compounding their limitations. Turbochargers do not have the horsepower requirements inherent in PD blowers; however, turbochargers suffer from low-end boost problems wherein the flow of exhaust at lower engine rpm is not sufficient to power the turbine as desired. Additionally, because turbochargers are not directly driven, they suffer from undesirable “lag” problems. Furthermore, both PD blowers and turbochargers are adiabatically inefficient relative to centrifugal superchargers in terms of boost created relative to flow. Prior art centrifugal superchargers have desirably less horsepower requirements than PD blowers and do not suffer from the lag problems of turbochargers. However, given their linear boost curve, these centrifugal superchargers either suffer from low end boost problems or high end boost problems. That is to say, if optimal boost is provided at higher engine rpms, the boost provided at lower engine rpms is significantly less than optimal. Conversely, if the typical centrifugal supercharger is set up to provide optimal boost at lower engine rpms, it will provide more boost than the engine can handle at higher engine rpms.
In powered vehicle applications, such as automotive applications, the problems and limitations detailed above are further compounded by the variable nature of the internal combustion engine. An internal combustion engine, when off idle, operates at an rpm that varies over a rev range. As the rpm varies, so does the normal flow of induction fluid through the engine, as well as the pressure of the induction fluid at the intake manifold. Additionally, as the rpm of the crankshaft varies, so does the rotational speed of any compressor impeller directly driven thereby. Furthermore, as the powered vehicle changes altitude, the pressure of ambient air at the forced air system's inlet varies (relative to normal atmospheric pressure at sea level, or 14.7 psi). However, the ideal target boost for automotive compressor applications remains constant (e.g., 10–12 psi of boost) over the entire rev range despite the varying conditions.
There have been prior art efforts to solve one or more of the above identified problems by using two compressors in combination to supply forced induction fluid to an engine. These prior art efforts fall into two general categories. First, it is known in the art to augment a turbocharger with a second compressor (either another turbocharger or a supercharger). In these prior art augmented turbocharger systems, the second compressor is only used below the powerband, typically to reduce lag problems, and only the turbocharger is used throughout the powerband. These augmented turbocharger systems utilize the second compressor only in series, or only in parallel (but typically dump the induction fluid compressed by the turbocharger below the powerband), and clutch the second compressor throughout the powerband. Secondly, it is known in the art to utilize two superchargers only in series, or only in parallel, to provide induction fluid to an engine. These dual supercharger systems are limited to constant speed engines, such as an airplane engine, and all utilize a variable speed drive, such as a clutch, to bypass one or both superchargers when the airplane is operating at lower altitudes.
These prior art dual compressor systems are problematic and subject to several undesirable limitations. For example, all of these prior art systems require a variable speed drive for at least one of the compressors. These variable speed drives are undesirable in that they are part intensive, have inherent mechanical limitations, and require wear-intensive parts subject to failure brought on by heat degradation. Additionally, the augmented turbocharger systems require undesirable part intensive valving in addition to at least one dump gate and are still limited by most of the problems inherent in turbochargers, such as relatively constant flow limitations. The dual supercharger systems are further limited to relatively constant speed engine applications and are not well suited for automotive applications. Accordingly, there is a need for an improved forced air induction system that is both operable to supply substantially constant target boost over the entire rev range of a variable rpm engine that does not suffer from the problems and limitations detailed above.